As dimensions of semiconductor devices shrink, isolation techniques have had to move from conventional local oxidation of silicon ("LOCOS") isolation techniques to silicon trench isolation. In fabricating an isolation structure for silicon trench isolation, a trench is created in a silicon substrate. The trench is then filled with an oxide to provide isolation.
In order to create the trench, a thin layer of oxide and a thicker layer of silicon nitride are grown and deposited, respectively, on the semiconductor substrate. Photoresist is then used to pattern the semiconductor, exposing areas in which the trench will be formed. The silicon nitride and oxide layers are then etched. The underlying semiconductor is then etched and the photoresist is stripped. In some conventional systems, an additional layer oxide is grown on the surface of the trench.
Although the conventional process discussed above is capable of providing a trench, the profile of the trench cannot be precisely controlled and tuned. For example, the conventional process results in comers at the base and at the top of the trench being relatively sharp. Consequently, device performance is adversely affected. In addition, the walls of the trench formed using a conventional process are relatively vertical. For narrow trenches, this vertical profile also makes filling the trench more difficult because it is more difficult for subsequently deposited materials to fill the trench. In addition, it may not be possible to form narrow shallow trenches which would also be wide enough to isolate structures on the semiconductor, particularly from charges which tunnel through the oxide filler within the trench. Moreover, the conventional process results in corners at the upper portion of the trench being relatively sharp and, therefore, high stress points. During a subsequent oxidation cycle, these corners can inhibit the formation of a good quality oxide. The lack of a good quality gate oxide adversely affects the device performance and reliability.
Accordingly, what is needed is a system and method for more accurately controlling the profile of a trench. In particular, control over the shape of the corners and incline of the trench walls is desirable. The present invention addresses such a need.